1. Field of the Invention
The invention relates to fluid delivery systems. More particularly, the present invention relates to monitoring the resistance to fluid flow in a fluid delivery system infusing fluid to a patient.
2. Description of Related Art
There are a variety of situations where fluid is infused to a patient. Applications of fluid delivery systems include (but are by no means limited to) intravenous infusion, intra-arterial infusion, infusion of enteral solutions, infusion of medication to the epidural space, and diagnostic infusion to determine vascular characteristics of the arterial, urinary, lymphatic, or cerebrospinal systems.
Fluid delivery systems for infusing fluid to a patient typically include a supply of the fluid to be administered, an infusion needle or cannula, an administration set connecting the fluid supply to the cannula, and a flow control device, such as a positive displacement infusion pump. The administration set typically comprises a length of flexible tubing. The cannula is mounted at the distal end of the flexible tubing for insertion into a patient's blood vessel or other body location to deliver the fluid infusate to the patient. The flow control device often is a peristaltic-type pump that acts on the flexible tubing to force the fluid through the tubing of the administration set to the cannula and into the patient. One commonly used flow control device is a linear peristaltic type pump having several cams and cam-actuated fingers that sequentially occlude portions of the flexible tubing along a pumping zone to create a moving zone of occlusion.
During an infusion procedure, events may occur that interfere with proper delivery of fluid to the patient, such as an occlusion of the administration line. It is desirable to detect these events as soon as possible so that they can be remedied.
A common technique for detecting such events and for evaluating fluid delivery system status is to monitor the pressure in the administration set. Variations in pressure can indicate problems with fluid delivery. For example, an increase in pressure over a selected threshold may indicate an occlusion in the system. Similarly, a drop in pressure can indicate an empty fluid supply or other fluid delivery system fault.
A problem with determining fluid delivery system status by monitoring pressure alone is the slow speed at which pressure can build when the system is operating at a low flow rate. At low flow rates, the energy per unit time introduced into the flow path is very small. Accordingly, it may take a considerable amount of time for the pressure to build up enough to exceed a threshold and indicate an occlusion. Additionally, with a relatively low pressure threshold, patient movements such as coughing, sneezing, and sitting up can cause the pressure to momentarily exceed the pressure threshold, thus creating a false alarm of a fluid delivery system fault. Another problem with monitoring pressure alone occurs when the delivery cannula becomes mislocated within the interstitial tissue matrix, causing a rise in pressure. The amount of resulting pressure rise is dependent upon flow rate. For example, at a flow rate of 10 ml/hr, the rise in pressure is typically only about 10 mm Hg; at a flow rate of 2 ml/hr, the rise in pressure is typically only about 2 mm Hg. Such small relative changes are difficult to detect from instantaneous pressure readouts, or even from pressure trends, because of the presence of other sources of change, such as patient movement, as well as venous pressure, elevation of the system components, and the flow rate itself.
As has been noted in U.S. Pat. No. 4,898,576 to Philip, the measure of the resistive part of the fluid line impedance can be used to monitor the condition of the fluid line. One technique used in actively monitoring the resistance, rather than merely waiting for pressure to build up, is the alteration of the flow rate. The change in the pressure over the change in the flow rate has been found to accurately indicate the resistive part of the fluid impedance in the system when adequate time is allowed for the pressure to reach equilibrium at each rate. This technique has been found to be effective at higher flow rates with their accompanying higher pressures. A change in these higher flow rates is accompanied by a rapid and measurable change in pressure. Because of the rapid pressure response to the flow rate changes, the flow rate can be varied about the selected flow rate without any significant clinical effect on flow uniformity.
However, at lower flow rates, the clinical requirement of flow rate uniformity restricts the magnitude of the changes to the flow that can be imposed on the fluid line. It is thus undesirable to alternate between different flow rates to obtain different pressure responses for determining resistance due to the detrimental effect on flow uniformity the flow changes would have as well as the relatively long length of time required to obtain those pressure responses.
Various models of pressure and resistance can allow accurate resistance measurements. For example, as described in U.S. Pat. No. 5,087,245 to Doan, which is incorporated herein by reference, a technique for determining flow resistance which allows for a non-linear relation between pressure and flow and a time-varying impedance (resistance and compliance) involves inducing a bolus of fluid in the infusion system and monitoring the resulting pressure wave and the pressure decay response. Injecting a known quantity of fluid causes a resulting pressure wave that then decays to an equilibrium pressure. Using the equilibrium pressure and the pressure decay response, fluid resistance can then be determined even when a non-linear relation between flow and pressure exists and when the impedance (resistance and compliance) are time-varying via the following equation: ##EQU1## where: .intg.F(t) dt=Q=the known delivered quantity of fluid, P(t)=the change in pressure over time,
P.sub.o =the equilibrium or offset pressure, PA2 A.sub.p =the area under a pressure response waveform, and PA2 A.sub.f =the area under a fluid flow waveform.
Thus, by injecting a known quantity of fluid through the infusion system, monitoring the resulting pressure as it decays to an equilibrium pressure, and determining an integral of the difference between the equilibrium pressure and the pressure response, the resistance to fluid flow can be determined. However, after the known quantity of fluid is injected, further quantities of fluid can not be injected through the system (i.e., further flow steps are not initiated) in order for the pressure to decay to the equilibrium pressure. In some situations, such as where the fluid resistance is relatively high, a relatively long pause in fluid injection may be necessary to allow the pressure to reach equilibrium pressure. Depending on the particular application, such long delays between fluid flow steps may be undesirable.
As set forth in pending U.S. patent application Ser. No. 08/305,904, pseudo-random binary sequence (PRBS) codes have been used to effectively eliminate the delays in reaching equilibrium pressure by creating "virtual" waveforms. However, due to the high processing requirements of PRBS coding and decoding procedures, PRBS codes have only been used in combination with linear and non-time-variant models of pressure and resistance, such as the following equation: ##EQU2## Where F=Flow rate, P=Pressure, and
Resistance and Compliance are stationary values (i.e., values that do not vary with time or flow).
Such a linear and non-time variant estimation technique is relatively accurate over fluid resistances between 0 to 1500 fluid ohms (where 1 fluid ohm=1 mm Hg per liter per hour). However, because fluid impedance (i.e., resistance and compliance) is in actuality time-variant and non-linear, the above-cited estimation technique has reduced accuracy where the fluid resistance exceeds 1500 fluid ohms.
Note that fluid resistance is a part of the total system fluid impedance. Fluid impedance is a function of the system compliance, inertance, and resistance.
The causes of the non-linear, time-variant resistance relationship include the viscoelasticity of the flexible tubing, which slowly contracts following application of a positive pressure transient. Additionally, the biochemical and rheological aspects of the patient's fluid system, such as the blood flow in the human body, further complicate the pressure/flow relationship.
There are several applications of parenteral infusion systems where both low flow rates and high resistances are encountered. For example, infusing parenteral liquids into small children and infants, and particularly into premature infants, can involve low flow rates and high resistances.
An additional problem with monitoring fluid infusion systems is caused by various "noise" sources that can degrade the pressure monitoring. Such noise sources include movement of the fluid infusion system and patient movement such as breathing and ambulation. Additionally, using more than one pump and administration set to inject fluid through a single fluid delivery system cannula can introduce noise that can interfere with individually monitoring the flow in the various administration sets.
Hence, those skilled in the art have recognized a need for a fluid delivery monitoring system that can detect a fluid delivery fault condition faster and with improved specificity than prior systems at low flow rates. There is also a recognized need for a system that can compensate for the existence of offset pressure while maintaining clinically acceptable flow patterns, and that can detect partial or "soft" occlusions that may result in pressure changes that are too small to be noticed through conventional pressure monitoring systems. Additionally, it has also been recognized that there is a need for a system that is less sensitive to other sources of pressure changes in the conduit such as those caused by other pumps on the same fluid line. It is further desirable to have a system that is accurate over a wide range of resistances and that is less sensitive to noise effects. The present invention satisfies these needs and others.